Conventionally, a semiconductor device including a semiconductor substrate and a sensor unit bonded to the semiconductor substrate through an insulation film has been known, for example as described in a patent literature 1. In the semiconductor device, the sensor unit is provided with a sensing part. The sensor unit is bonded to the semiconductor substrate such that the sensing part is sealed between the sensor unit and the semiconductor substrate.
In such a semiconductor device, the semiconductor substrate is formed with a recessed portion at a position corresponding to the sensing part so as to restrict the sensing part from contacting the semiconductor substrate. The insulation film is disposed between the sensor unit and the semiconductor substrate to provide electric insulation between the sensor unit and the semiconductor substrate.
The semiconductor device is, for example, produced in a following manner.
First, a first semiconductor wafer is prepared. The sensing part is formed in each of chip formation regions of the first semiconductor wafer, the chip formation region providing the sensor unit when the first semiconductor wafer is divided into chips. Also, a second semiconductor wafer, which provides semiconductor substrates when being divided into chips, is prepared. The recessed portion is formed in the second semiconductor wafer at a position corresponding to each of the sensing parts. Then, the second semiconductor wafer is thermally oxidized such that a thermal oxide film is formed on a surface of the second semiconductor wafer, which is to be opposed to the first semiconductor wafer.
Further, the first semiconductor wafer and the second semiconductor wafer are stacked such that the first semiconductor wafer is boned to the thermal oxide film of the second semiconductor wafer. Thus, a stacked wafer is produced. Then, the stacked wafer is divided into chips. The semiconductor device is provided by the chip.
In the method described above, the thermal oxide film is formed after the recessed portion is formed in the second semiconductor wafer. Therefore, the oxidation is accelerated at a position adjacent to the opening perimeter of the recessed portion due to stress being concentrated. As a result, a thickness of the thermal oxide film is increased at the position adjacent to the opening perimeter of the recessed portion than a remaining portion, such as a region on a periphery of the opening perimeter (e.g., see a non-patent literature 1). In other words, the thermal oxide film is formed as it rises at the position adjacent to the opening perimeter of the recessed portion.
If the first semiconductor wafer is bonded to the second semiconductor wafer through this thermal oxide film, only the portions of the thermal oxide film adjacent to the opening perimeters of the recessed portions are bonded to the first semiconductor wafer. That is, only the risen portions of the thermal oxide film are bonded to the first semiconductor wafer. As a result, a bonding property between the thermal oxide film (second semiconductor wafer) and the first semiconductor wafer is insufficient. Namely, when the stacked wafer is divided into the chips, the bonding property between the sensor unit and the semiconductor substrate is insufficient.
<Patent Literature 1>
Japanese Patent Application Publication No. 2011-199301
<Non-Patent Literature 1>
T. Suni, K. Henttinen, J. Dekker, H. Luoto, M. Kulawski, J. Makinen, and R. Mutikainen, “Silicon-on-Insulator Wafers with Buried Cavities”, Journal of The Electrochemical Society, 153 (4) G299-G303 (2006)